Lithography is a process for producing a pattern on a semiconductor wafer. The pattern is produced by first exposing a pattern etched into a mask onto a semiconductor wafer coated with a resist material. The projected image of the pattern changes the composition of the resist material on the semiconductor wafer which is then removed to leave a matching pattern on the semiconductor wafer for further processing.
Typically, the masks used in lithography each include a substrate which is coated with a film. Depending upon the particular lithography application, the mask needs to satisfy several different requirements. The challenge is in finding a material or materials for use as the film which will satisfy these requirements.
For lithography at or below 0.20 .mu.m and for use at wavelengths at or below 300 nm, an attenuated phase shift mask must have certain optical properties. These optical properties include a transmission between about 1% and 90%, an appropriate phase shift, e.g. a phase shift of about 180 degrees, a refractive index of between about 1.5 and 30, an extinction coefficient between about 0.1 and 1.0 and reflectivity below about 20 percent.
Additionally, the attenuated phase shift mask must have suitable plasma etch characteristics with selectivity to the underlying substrate and to the resist material. In other words, the film on the substrate of the attenuated phase shift mask must be made of a material or materials which can be etched to form the pattern to be replicated on semiconductor wafers without significant loss to the underlying substrate or to the resist material.
Since the intended application of the attenuated phase shift mask is for use with energetic ultra violet (UV) radiation from an excimer laser, the film on the substrate of the attenuated phase shift mask must be able to withstand both short and long term exposure to the UV radiation without significant optical degradation. Typically, damage to the film from exposure to radiation from the excimer laser can be shown to correlate with spatial and temporal peak power density as well as maximum power density and total energy density. Since single laser pulse energy levels from the excimer laser for lithography at or below 0.20 .mu.m are typically low, damage to the attenuated phase shift mask generally results from total energy density from the excimer laser. Damage from long term or cumulative exposure can result from thermo-chemicals effects, the migration of defects, damage to microscopic defects, or surface particle formation. This damage can detrimentally effect the optical properties of the attenuated phase shift mask. Understoichiometric films on substrates are particularly vulnerable to damage from exposure to radiation.
As yet, an appropriate material or materials for use as the film on the substrate of the attenuated phase shift mask for lithography at or below 0.20 .mu.m and for use at wavelengths below 300 nm has not been found.